Some 7.4 million Americans last month were working part-time jobs because they couldn’t find full-time work. Many of them will have better luck as the economy improves, but others could be locked out by the changing nature of the U.S. economy, four Federal Reserve economists argue in a new analysis.

So-called “involuntary part-time employment” has fallen in goods-producing industries as factories ramped up production in recent years. But in the services sector, “the progress has been somewhat slower and the fraction working part-time due to slack conditions remains substantially elevated,” wrote Tomaz Cajner, Dennis Mawhirter, Christopher Nekarda and David Ratner.

“The higher prevalence of the latter type of involuntary part-time work in service-providing industries could be related to some intrinsic (technological) characteristics of these industries. For example, the leisure and hospitality and the retail trade industries both have shorter workweeks and a higher share of part-time workers, likely reflecting some organizational aspects of their production processes,” they wrote. “Ongoing transformation of U.S. employment away from manufacturing and towards services-providing industries could lead to a secularly-increasing share of (involuntary) part-time work going forward, although quantitatively the effect is expected to be modest.”

How modest? “Since 2000, the changing industry composition contributed a few tenths of a percentage point to the share of involuntary part-time work in the labor force,” they wrote in a footnote.

In any case, the economists wrote, the number of Americans who want a full-time job but can’t find one “remains unusually high” and “has led to a concern that there is an underbelly of labor market slack not well accounted for by the overall unemployment rate.”

That’s a concern shared by Fed Chairwoman Janet Yellen. “The unemployment rate is down, but not included in that rate are more than 7 million people who are working part time but want a full-time job. As a share of the workforce, that number is very high historically,” she said in Chicago on March 31.

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